Water Addition Plots Research Articles

Precipitation legacies amplify ecosystem nitrogen losses from nitric oxide emissions in a Pinyon-Juniper dryland.

Climate change is increasing the variability of precipitation, altering the frequency of soil drying-wetting events and the distribution of seasonal precipitation. These changes in precipitation can alter nitrogen (N) cycling and stimulate nitric oxide (NO) emissions (an air pollutant at high concentrations), which may vary according to legacies of past precipitation and represent a pathway for ecosystem N loss. To identify whether precipitation legacies affect NO emissions, we excluded or added precipitation during the winter growing season in a Pinyon-Juniper dryland and measured in situ NO emissions following experimental wetting. We found that the legacy of both excluding and adding winter precipitation increased NO emissions early in the following summer; cumulative NO emissions from the winter precipitation exclusion plots (2750 ± 972 μg N-NO m-2 ) and winter water addition plots (2449 ± 408 μg N-NO m-2 ) were higher than control plots (1506 ± 397 μg N-NO m-2 ). The increase in NO emissions with previous precipitation exclusion was associated with inorganic N accumulation, while the increase in NO emissions with previous water addition was associated with an upward trend in microbial biomass. Precipitation legacies can accelerate soil NO emissions and may amplify ecosystem N loss in response to more variable precipitation.

Summer precipitation limits plant species richness but not overall productivity in a temperate mesic old‐field meadow

AbstractQuestionWater availability is a primary regulator of plant productivity and species richness in arid and semi‐arid ecosystems, but its influence in other habitats is much less clear partly because experimental manipulations of water are relatively rare. How important is soil moisture availability relative to other key environmental factors in determining plant species biomass and diversity in temperate mesic grasslands?LocationAn old‐field meadow in southeastern Ontario, Canada.MethodsWe established a long‐term factorial field experiment to investigate how variation in soil water (achieved by rainout shelters and water additions), nutrients (NPK fertilizer additions), and above‐ground herbivory (deer fence exclosures), interact as regulators of plant production, species richness, and species composition. Above‐ground biomass for each species was determined in replicate plots (1 m2, n = 10) after five years of treatment applications.ResultsThe rainout shelters reduced community shoot biomass by 35% relative to ambient (control) plots, with no influence of the fertiliser. To our surprise, however, the fertiliser enhanced community biomass in the ambient and water addition plots to a similar extent. Species richness and Shannon diversity were increased by water addition, but reduced by the rainout shelters and by fertilisation. Total species richness across all replicate plots was 60% higher in the water addition treatment, and almost halved in the water‐reduced treatment. In contrast to the above, the exclosures had negligible impacts on these community variables.ConclusionsTogether, these results suggest that in old‐field meadow grasslands: (a) shoot productivity is primarily limited by soil nutrient supply except under drought conditions when water availability becomes the primary constraint; (b) the two main soil resources for plants — water and nutrients — affect productivity and species richness/composition in fundamentally different ways; and (c) future increases in summer soil aridity and anthropogenic nitrogen deposition may interact to reduce vegetation productivity and diversity in temperate mesic ecosystems.

Evolutionary responses of invasive grass species to variation in precipitation and soil nitrogen

SummaryGlobal climate models suggest that many ecosystems will experience reduced precipitation over the next century and the consequences for invasive plant performance are largely unknown. Annual invasive species may be able to quickly evolve traits associated with drought escape or tolerance through rapid genetic changes.We investigated the influence of 5 years of water and nitrogen manipulations on trait values in a southern California grassland system. Seeds from two annual grass species (Avena barbataandBromus madritensis) were collected from experimental plots and grown in a common environment over two generations. We measured 14 physiological, morphological, phenological and reproductive traits.Both species displayed phenotypic differences depending on the water treatment from which they were collected, but not depending on the nitrogen treatment. Both species displayed trait values characteristic of drought escape (e.g. earlier flowering inA. barbataandB. madritensis, lower water‐use efficiency inB. madritensis) when grown from the seeds collected from plots that experienced five years of reduced precipitation. Furthermore,A. barbataindividuals grown from the seeds collected from drought plots had higher reproductive output and higher photosynthetic performance than individuals grown from water addition plots, with individuals grown from ambient plots displaying intermediate trait values. Notably, we found no phenotypic variation among treatments for six root traits.Synthesis. Trait differences were observed following two generations in a common garden, suggesting that treatment differences were genetically based. This suggests that populations were responding to selection over the 5 years of water manipulations, a remarkably short time period. The rapid evolutionary responses observed here may help these two widespread invasive grass species thrive under reduced precipitation scenarios, which could have important implications for fire dynamics, invasive species management and native plant restoration in communities invaded by annual grasses.

Coupled response of soil carbon and nitrogen pools and enzyme activities to nitrogen and water addition in a semi-arid grassland of Inner Mongolia

Previous studies have demonstrated positive net primary production effects with increased nitrogen (N) and water availability in Inner Mongolian semi-arid grasslands. However, the responses of soil carbon (C) and N concentrations and soil enzyme activities as indicators of impacts of long-term N (urea) and water addition are still unclear. We tested the effect of 7 years of a N and water addition experiment on soil C, N, and specific soil-bound enzymes in a semi-arid grassland of Inner Mongolia. We determined concentrations of soil organic carbon (SOC) and soil total nitrogen (TN) in both the 0–10 and 10–20 cm soil layers. Concentrations of labile carbon (LC) and inorganic nitrogen (nitrate and ammonium), and soil pH were measured. Additionally, soil dehydrogenase (DHA), β-glucosidase (BG) and acid and alkaline phosphomonoesterase (PME) enzyme activities were determined in the 0–10 cm soil layer. SOC concentration in the 0–10 cm soil layer showed no response to N addition or N plus water addition, but increased with water addition alone by 0.3–15.7 %. N addition significantly increased nitrate by 46.0–138.4 % and ammonium by 19.0–73.3 % in the 0–10 cm soil layer, whereas water addition did not affect them. The activities of DHA and alkaline PME enzymes, as well as soil pH, in the 0–10 cm layer decreased with N addition, however water addition alone caused these enzyme activities to increase. Unlike the surface soil (0–10 cm), the lower soil layer (10–20 cm), was responsive to N and water addition in that SOC and TN concentrations decreased with N addition and increased with water addition. The accumulation of SOC and TN in N and water addition plots may be caused by the input of plant biomass exceeding SOC decomposition. Decrease in microbial activity, derived from decreased DHA and alkaline PME activities might result from suppression effects of lower pH and decreased microbial N supply. Water availability is proved to be more important than N availability for soil C and N accumulation in this semi-arid grassland.

The impact of precipitation change on nitrogen cycling in a semi‐arid ecosystem

SummaryClimatic change is altering ecosystem structure and function, especially in the southwesternUnitedStates where trees are near their physiological water stress threshold. In piñon‐juniper (Pinus edulis‐Juniperus monosperma;PJ) woodlands, increased drought is causing differential mortality of piñon resulting in an ecosystem that is becoming juniper dominated.Using a precipitation manipulation, we assessed how both increased and decreased precipitation altered ecosystem function beneath piñon and juniper. We predicted that changes in precipitation would alter nitrogen (N) availability and mineralization at the site. Further, we predicted that these responses would differ beneath piñon and juniper crowns due to plant‐level differences in transpiration andNuptake in response to drought.We found minimal interactions between tree species and the precipitation treatments onNcycling. However, across all years measured, soil nitrate decreased with increasing soil volumetric water content; a pattern that is likely due to reduced turnover in dry plots. In contrast, potential soil net‐nitrogen mineralization was reduced in water removal plots relative to water addition plots indicating that nitrogen cycling rates were slower under drought. Tree type also influenced nitrogen dynamics in this woodland. Across all 4 years, soilNavailability and potential soil net‐mineralization rates were higher in soils beneath piñon relative to juniper across all treatments. Interestingly, the observed shifts inNcycling were not reflected in the abundance ofNin microbial biomass or in ammonia‐oxidizing bacteria, which are responsible for nitrification. The observed patterns may be due to increasedNleaching from the soil during periods of increased rainfall or due to decreased microbial activity or plantNuptake when conditions are dry.The effect of precipitation change onNcycling may have long‐term consequences on the plant community in this semi‐arid ecosystem. Nitrogen concentrations are highest in the soil when water availability is low, thus whenNconcentrations are high, plants and microbes are relatively inactive and unable to use this resource.

Effects of experimental water addition depend on grassland community characteristics

Plant community biomass and composition on low-productivity soils, such as serpentine, may be more resistant to climate change because they host stress-tolerant species that may respond slowly to change. These communities also host a number of endemic taxa that are of special interest because of their narrow distributions. In a 3-year study, we experimentally tested the response of serpentine and non-serpentine communities to water addition in spring. We also compared the responses of endemics and generalists to water addition, with and without biomass (competitor) removal. In the non-serpentine grassland, peak biomass was significantly greater in the water addition plots compared with control plots, but this effect depended on the year. In the serpentine grassland, there was no effect of water addition on biomass. Survival, biomass, growth rates, and seed production of soil endemics and generalists were all significantly reduced by competition, but were unaffected by water addition. Overall, endemics tended to perform better in serpentine soil and generalists in non-serpentine soil, suggesting that soil is an important factor for the establishment and survival of endemics and generalists. For endemics, the effect of biomass removal was stronger on non-serpentine soil, but for generalists this effect was similar on both soils, indicating that competition can be important in low-resource habitats. In conclusion, our results suggest that low-fertility plant communities may be slow to respond to changes in precipitation compared to communities on more fertile soil.

When Wet Gets Wetter: Decoupling of Moisture, Redox Biogeochemistry, and Greenhouse Gas Fluxes in a Humid Tropical Forest Soil

Upland humid tropical forest soils are often characterized by fluctuating redox dynamics that vary temporally and spatially across the landscape. An increase in the frequency and intensity of rainfall events with climate change is likely to affect soil redox reactions that control the production and emissions of greenhouse gases. We used a 24-day rainfall manipulation experiment to evaluate temporal and spatial trends of surface soil (0‐20 cm) redox-active chemical species and greenhouse gas fluxes in the Luquillo Experimental Forest, Puerto Rico. Treatments consisted of a high rainfall simulation (60 mm day -1 ), afl uctuating rainfall regime, and a control. Water addition generated high temporal and spatial variation in soil moisture (0.3‐0.6 m 3 m -3 ), but had no significant effect on soil oxygen (O2) concentrations. Extractable nitrate (NO3 - ) concentrations decreased with daily water additions and reduced iron (Fe(II)) concentrations increased towards the end of the experiment. Overall, redox indicators displayed a weak, non-deterministic, nonlinear relationship with soil moisture. High concentrations of Fe(II) and manganese (Mn) were present even where moisture was relatively low, and net Mn reduction occurred in all plots including controls. Mean CO2 fluxes were best explained by soil C concentrations and a composite redox indicator, and not water addition. Several plots were CH4 sources irrespective of water addition, whereas other plots oscillated between weak CH4 sources and sinks. Fluxes of N2 Ow ere highest in control plots and were consistently low in water-addition plots. Together, these data suggest (1) a relative decoupling between soil moisture and redox processes at our spatial and temporal scales of measurement, (2) the co-occurrence of aerobic and anaerobic biogeochemical processes in well-drained surface soils, and (3) an absence of threshold effects from sustained precipitation on redox reactions over the scale of weeks. Our data suggest a need to re-evaluate representations of moisture in biogeochemical models.

Methodology and performance of a rainfall manipulation experiment in a piñon–juniper woodland

Climate models predict that water limited regions around the world will become drier and warmer in the near future, including southwestern North America. We developed a large‐scale experimental system that allows testing of stand level impacts of precipitation changes. Four treatments were applied to 1600 m2 plots (40 m × 40 m), each with three replicates in a piñon pine (Pinus edulis) and juniper (Juniper monosperma) ecosystem. These species have extensive root systems, requiring large‐scale manipulation to effectively alter soil water availability. Treatments consisted of: (1) irrigation plots that receive supplemental water additions, (2) drought plots that receive 55% of ambient rainfall, (3) cover‐control plots that receive ambient precipitation, but allow determination of treatment infrastructure artifacts, and (4) ambient control plots. Our drought structures effectively reduced soil water potential and volumetric water content compared to the ambient, cover‐control, and water addition plots. Drought and cover‐control plots experienced an average increase in maximum soil and ground‐level air temperature of 1–4°C during the growing season compared to ambient plots, and concurrent short‐term diurnal increases in maximum air temperature were also observed directly above and below plastic structures. Our drought and irrigation treatments significantly influenced tree predawn water potential and canopy transpiration, with drought treatment trees exhibiting significant decreases in physiological function compared to ambient and irrigated trees. Supplemental irrigation resulted in a significant increase in both plant water potential and canopy transpiration compared to trees in the other treatments. This experimental design allows manipulation of plant water stress at the tree/stand scale, permits a wide range of drought conditions, and provides prolonged drought conditions comparable to historical droughts in the past—drought events for which wide‐spread mortality in both these species was observed.

Responses of native and non-native Mojave Desert winter annuals to soil disturbance and water additions

Habitat modification (i.e., disturbance) and resource availability have been identified as possible mechanisms that may influence the invasibility of plant communities. In the Mojave Desert, habitat disturbance has increased dramatically over the last 50 years due to increased human activities. Additionally, water availability is considered to be a main limiting resource for plant production. To elucidate the effects of soil disturbance and water availability on plant invasions, we created experimental patches where we varied the levels of soil disturbance and water availability in a fully crossed factorial experiment at five replicated field sites, and documented responses of native and non-native winter annuals. The treatments did not significantly affect the density (seedlings m−2) of the non-native forb, Brassica tournefortii. However, the relationship between silique production and plant height differed among treatments, with greater silique production in disturbed plots. In contrast to Brassica, density of the non-native Schismus spp. increased in soil disturbed and watered plots, and was greatest in disturbed plots during 2009 (the second year of the study). Species composition of the native annual community was not affected by treatments in 2008 but was influenced by treatments in 2009. The native forb Eriophyllum sp. was most dense on water-addition plots, while density of Chaenactis freemontii was highest in disturbed plots. Results illustrate that habitat invasibility in arid systems can be influenced by dynamics in disturbance regimes and water availability, and suggest that invasiveness can differ between non-native annual species and among native annuals in habitats undergoing changing disturbance and precipitation regimes. Understanding the mechanistic relationships between water availability and non-native plant responses will be important for understanding the effects of shifting precipitation and vegetation patterns under predicted climate change in arid ecosystems.

Vegetation dynamics following resource manipulations in herb-rich woodland

Changes in species richness, turnover, composition and above-ground biomass of herb-rich woodland were documented following fertilizer application and water addition over three growing seasons. Addition of fertilizer significantly reduced species richness relative to unmanipulated control and water addition plots after 3 years. This change coincided with significant increases in biomass, which were largely due to increased growth of exotic annual grasses. The reductions in richness observed in the fertilized plots were a consequence of both lower rates of local colonization and enhanced rates of local extinction of the resident species. Species loss was not random; native species were lost after nutrient addition whereas exotic species were not. Nutrient limitation was more important for species coexistence in these communities than was water availability.

Ecological resistance, seed density and their interactions determine patterns of invasion in a California coastal grassland

Relatively little experimental evidence is available regarding how ecological resistance and propagule density interact in their effects on the establishment of invasive exotic species. We examined the independent and interactive effects of neighbour cover (biotic resistance), winter vs. spring water addition (abiotic resistance) and seed density on the invasion of the European perennial grass Holcus lanatus into a California coastal grassland dominated by exotic annual grasses. We found that decreased competition from resident exotic grasses had no effect. In contrast, increased late-season water availability eroded the abiotic resistance offered by naturally dry conditions, facilitating invasion. Finally, watering treatment and seed density interacted strongly in determining seedling survival: while seedling mortality was close to 100% in ambient and winter water addition plots, survivor numbers increased with seed density in spring-watered plots. Thus, decreased abiotic resistance can amplify the effect of increased propagule density on seedling establishment, thereby increasing the likelihood of invasion.